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THE GEOLOGY OF THE MUSICK POINT

- BUCKLANDS' BEACH AREA

by J. Chappell

ABSTRACT

Two periods of deformation and disruption of the beds of the Waitemata Group are suggested; an early period of more complex deformation brought about by sliding and slumping was followed by a later period of gentle folding and normal faulting accompanying uplift. It is proposed that much of the large scale slump deformation was brought about by the emplacement of a bed of unstratified grit, thirty to forty feet thick. This grit probably originated as a large -sca le gravity-gliding mass from a western volcanic chain and was deposited as a submarine lahar flow. An unstable shelf environment of deposition is suggested. Most of the sedimentary material seems to have been derived from a eastern graywacke landmass, and a w e s t ­ern andesitic volcanic chain.

INTRODUCTION

Musick Point is the northernmost extremity of the Bucklands 1 Beach - East ­ern Beach peninsula, which forms the eastern side of the Tamaki Estuary. The bulk of the rocks in the area belong to the Waitemata Group, which here includes a bed of unstratified grit, 30 to 40 feet thick. This bed is lithologically similar to the Parnell Grit, but as it is not known whether it is a correlative of the grit at the type locality at Judges Bay, it will be referred to as the East Tamaki grit. A total thickness of five hundred feet of strata of the Waitemata Group is present in the area. The beds are well-bedded alternating sub graywacke sandstones and s i l t -stones with no stratigraphic breaks in the sequence. The East Tamaki Grit o ccurs 125 feet below the top of the section.

A small area of Quaternary beds l ies over the folded and faulted rocks of the Waitemata Group at the northern end of Bucklands' Beach.

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STRUCTURE

(i) Faulting:

The faults fall easily into two c lasses , the f irst c lass contains major north-east striking normal faults, dipping south at 6 0 ° - 80° and having downthrows of as much as 110 feet. Five of these appear on the map. Poor ly developed shear zones, up to three feet wide, mark the planes of the larger faults, and drag effects are reflected in the adjacent beds up to one chain f rom the fault plane.

These major faults are the youngest tectonic features of the area, disrupting all other structural features of the Waitemata Group. They are post-depositional and may be correlated with tectonic uplift of the Waitemata sediments.

The second c lass of faults includes all the intraformational and slump faults, formed by compaction and submarine slumping during deposition of the sediments. They include normal and reverse faults which range in length from only a few yards to tens of chains, with displacements up to 30 feet or m o r e . These faults character­istically show numerous splinters, and thinning and thrusting effects without any great drag or shearing are common. These all give a general indicated direction of slumping and sliding f rom W. S. W. to E. N. E. (See figure 4. ) .

(ii) Fo ld ing :

Fold axes trend north-south in the area, and the nature of the folding varies markedly. It is suggested that the folding, like the faulting, can be divided into two groups; folding of tectonic origin, and folding due to submarine slumping. All fold axes appear on the map.

A gentle but persistent syncline runs f rom Musick Point to Eastern Beach. At the latter locality it is mere ly a gentle depression in the cres t of a larger anticlinal warp. J This fold affects all the beds in the section; the dip of the l imbs is always les than 15° except where slump folds are superimposed and the fold plunges north at abou 8° . This fold is tectonic in origin, as may be the anticline of the western shore platform, and the gentle flexure in the strata on the eastern side. Furthermore , tectonic compress ion may have rejuvenated movement along some of the ear l ier - form­ed slump folds and thrusts.

The folding preceded faulting, and probably took place before uplift. Faulting followed, with the main stress direction changing f rom a lateral compress ion to a vertical uplift.

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Th e more complex folds in the area are those which have been caused by sub­marine slumping at the time of deposition. These are divisible into two c l a s s e s : small scale and large scale structures.

Small scale structures are of the type generally recognised as being character ­istic of subaqueous slumping and s l id ing: - intraformational contorted beds, scour casts inclusions of one bed in the next, thinned beds, and complex f lame-l ike structures (not density-current formed flame structures) as well as clastic dykes.

Several relatively complex large -sca le structures occur in the area; these are pictured in figures 1, 2, and 3 and they are discussed in turn below.

Figure 1.

This recumbent fold is very similar to complex folds figured by Turner and Bartrum (1929), which were considered by them to be of tectonic origin. Kuenen (1949) redescribed these folds and suggested that they were formed as a result of sub­marine slumping. Several features of the fold in 1 support this mode of origin, i. e. the very short axial length of the fold (20 chains maximum), the complexity of the o v e r ­ridden beds , and the absence of the East Tamaki Grit f rom the core of the fold. Strati graphic ally, the Grit should occur here, but instead a poorly bedded tuffaceous sandstone lies in this position, being interposed between the grit and the beds which normally lie above it.

Figure 2 and 3.

It is suggested that the large anticline shown in 2 has been formed by subaqueous slumping, for the following r e a s o n s : -

First ly , the steeper western limb is thinned drastically, and this thinning is not accompanied by any shearing; secondly, the fold has a short axial length (40 chains at the most) , and is terminated at its southern end by the slump balls shown in 3; and thirdly, this fold appears to be the culmination of smal ler -sca le features which become more abundant in the shore platform and cliff towards the locality of the anticline.

The western shore platform also contains evidences of slumping, which include clastic dykes, a clastic " r i n g - d v k e " (near the north end of Bucklands' Beach), long zones of splintered faults, and inclusions of one bed in the next.

Thus one can conclude that the more intense deformation of these beds of the Waitemata Group is due to submarine slumping, and may have been caused by the o v e r ­riding of a large sliding mass . If this is so, the overriding mass has travelled from west to east. This suggestion is supported by the north-south alignment of the slump fold axes, by the strong thinning of the western limb of the anticline of figure 2, and by intraformational faults which indicate thinning by slumping (figure 4) .

SECTION A A ' , SCALE ? j CHAINS 7 0 THE INCH.

PARNELL GRIT

SECTION B B ' . SCALE ? CHAINS TO THE INCH.

E .

SECTION C C SCALE 5 CHAINS TO THE INCH.

SECTION D D ' , SCALE 5 CHAINS TO THE INCH.

THE GEOLOGY OF

MUSICK POINT.

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THE NATURE AND ORIGIN OF THE EAST TAMAKI GRIT

The grit bed varies from 30 to 40 feet thick. It is a massive unbedded stratum, comprised of volcanic grits and tuffs. The coarsest fragments occur near the base, and have a maximum size of 2-3 cm. diameter. The majority of the grit is fine grained, and the matrix shows a moderate degree of sorting (figure 5 )• The grit fragments are predominantly andesitic in nature, but many finer fragments of silt-stone are present. Abundant hypersthene, together with lesser hornblende and ore makes up the bulk of the heavy mineral concentrate. The bed is well jointed; the joints are irregular and filled with limonite encrusting on calcite. The basal contact is irregular , showing scour casts and inclusions of the underlying bed.

The scour casts are a series of aligned troughs, about two feet wide and 9 to 15 inches deep. They are best exposed in the cliff 12 chains south of the point on the eastern side of the peninsula, and again 30 chains south of the point on the same side, where they are aligned in a SW - NE direction.

Several suggestions have been put forward to explain the origin of the grit horizons which occur within the Waitemata Group; these are recapitulated here.

Mulgan (1901) correlates all beds of the Parnell Grit fac ies , and suggests that most of the material could have been derived from the Waitakere region, with some local volcanic action giving rise to the coarser facies. Fox (1901), on the other hand, considered the grit to have been derived from the Coromandel volcanic chain. Turner and Bartrum (1929) believed that local submarine volcanoes were responsible for their emplacement. However, Kuenen(l949) has suggested that the grits are submarine lahar flows that originated on a westerly volcanic chain (in the Waitakere area), and this is the explanation favoured for the origin of the East Tamaki Grit, as the grit c losely resembles a lahar mudflow in that it is unbedded, rather variable in thickness and contains inclusions of the underlying beds in its lower parts. All of the features listed above support this mode of origin.

THE WAITEMATA SEDIMENTS

The rocks of the Waitemata Group consist of well bedded alternating sub-gray-wacke sandstones and siltstones. Most of the sandstones are massive , although many of the thinner beds show graded bedding to some degree. Current bedding, where present, is usually of the current-ripple type, although tractive current bedding is seen occasionally; small ripple marks, flame structures and load casts occur frequent­ly throughout the column. There is a certain amount of thinning and thickening of the beds; the whole column seems to thin southward at the rate of 8% per mile , but

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individual beds may thin in either direction and some lens out completely.

The sandstones contain a rather rich heavy mineral suite, which includes the fo l lowing: - abundant zircon, hypersthene, and ore minerals ; moderate amounts of apatite, epidote and biotite, and sparse to rare sphene, hornblende, garnet, titanaugite and clinozoisite. Grain size histograms of the sandstones and siltstones are given in figure 5. The amount of feldspar and clastic material in the sandstones, and the relatively poor degree of sorting justifies their classification as sub-graywackes, (Pettijohn, 1957). The siltstones which alternate with them are sometimes highly sulphurous and contain abundant carbonaceous fragments. No macrofoss i l s were found.

A C - M graph (figure 6 ) , drawn on a rather inadequate number of points, suggests that tractive currents have been instrumental in depositing a certain amount of the sediment.

CONCLUSIONS

A landmass of relatively high relief lay to the east during Oligocene times. A second shoreline probably lay c lose to the western margin of the present Waitakere Ranges. The Eastern land was largely graywacke. The continental shelf type of basin, in which the Waitemata sediments were deposited, was downsinking and some­what prone to earthquakes, which may have been associated with the intense andesitic vulcanism that was occurring in the west. Tractive currents flowed across the shelf in an easterly direction, punctuated by frequent, northeasterly-flowing turbidity currents. These latter may have arisen on large deltas which flanked the western volcanic chain. At least one, and possibly more tremendous lahars flowed off this western chain, spreading out as high-density mud-flows across the shelf and causing minor contortion, and differential compaction of the underlying beds may have been quite profound in that the overriding movement from west to east may have produced the large-sca le slump structures in the beds below which also show west-easterly trend.

Uplift, accompanied by normal faulting, followed during post-Altonian times. Fluctuating Pleistocene sea levels cut terraces at 110' - 130' and 10' - 15' above sea level and deposited sediments, some of which outcrop at the north end of Bucklands 1

Beach. Recent seas cut extensive shore platforms, especially on the grit bed where it occurs at sea level.